In gear shift control mechanisms, there is typically a jaw clutch which moves between two positions corresponding to two different gear ratios of the transmission. Shifter forks are provided for actuation by a shifting mechanism to move the jaw clutches to the desired position and activate the various gear ratios. Where there are a plurality of gear ratios involved, a number of systems have included a separate shift rail for each shifter fork. The rail is free to move axially with its supports, and each rail has a separate shifter with slots into which a shift finger or lever can be accepted. By appropriate selection of each shifter by the finger and subsequent fore or aft movement, the various transmission jaw clutches can be engaged. Each rail is detented at three positions, the neutral position and a position corresponding to each different gear ratio on either side of the neutral position.
With this system, a bell-crank arrangement may be needed for providing a progressive shift pattern. A problem with this system is the large number of parts required to change gear ratios, as well as, maintain acceptable tolerances during a shifting operation. The invention described herein obviates the need for the large number of parts which has characterized prior systems as discussed above and as a result eliminates the accompanying tolerance problems. In addition, the invention provides for a minimum shifting schedule which, in addition to reducing the tolerance problem, enhances the feel and acceptability to the operator.
Specifically, as will be pointed out in more detail in the description of the preferred embodiment, the invention disclosed herein includes a stationary rail on which shifter forks are moveable fore and aft as a result of rotative actuation of a selector shaft. Intermediate the selector shaft and the shifter fork there is a mechanism including a series of cams and levers for transposing the rotative motion of the selector shaft to the linear motion needed for movement of the shifter forks along the single stationary rail. Once a cam and lever is selected by movement of the selector shaft along its longitudinal axis, the selector shaft is then rotated whereby the cam, lever mechanism drives the shifter fork along the stationary rail in one direction or the other to achieve the desired gear ratio. By using a stacked system with the apparatus described above, minimum shift schedule is achieved and high tolerances can be maintained.
In addition to having a relatively small number of parts, this mechanism can use stamped components requiring little or no subsequent machining in place of presently utilized forgings. This results in a compact, reduced weight mechanism having low inertia and improved "feel". The single rail requires less clearance between main or counter shaft gears. On transmissions of the multi-counter shaft variety this compactness is significant because it places the shifter forks closer to the output or mainshaft resulting in shorter forks which will be less prone to wearproducing deflections. Manufacturing efficiencies are gained by not having to provide multiple, bearing quality alignments or fits between the rails and supports.
Although there have been previous attempts to employ a single rail for a number of shifter forks, these have not achieved the attributes attendant the construction and mode of operation of the invention described herein. Consequently, these other single rail devices suffer from disadvantages in tolerances, shifting schedule, versatility and operator feel, which the instant invention has overcome.
An example of a single rail construction, over which the invention described herein has improved, is shown in U.S. Pat. No. 3,544,047 to Stott (hereinafter referred to as Stott). Stott relates to a selector mechanism having the striker forks slideably mounted on a single striker shaft where, according to the patentee, the striker members can be grouped more closely together. The selector mechanism may also include an interlock portion to prevent movement of striker members other than the selected one. The selector shaft is mounted for rotary movement about a longitudinal axis, as well as for movement in the longitudinal direction. A longitudinal blade-like extension has first and second selector portions in the form of recesses circumferentially spaced around the selector shaft to engage selected striker forks. In operation a gear shift lever is actuated to effect rotary movement of the selector shaft and bring these selector portions in alignment with corresponding selector portions on the striker members. The gear shift lever is subsequently operated to produce longitudinal movement of the selector shaft thereby moving the selected striker member, engaged by its respective selector portion, through a change in gear ratio selection.
The Stott configuration cannot obtain the minimum shift schedule or the high tolerance of the cam and lever mechanism of the present invention. In addition there is also a certain versatility achieved by this invention which was not obtained by devices such as those shown by Stott. This versatility is the ability to maintain a progressive shift pattern for the operator with various arrangements of the gears. For example, the alignment of the gears and the shifting forks may be such that the shifting between ratios results in a shift pattern or schedule that is not progressive. By this, it is meant that the shift from first to second may be in one direction, but the shift from third to fourth may be in the opposite direction. This produces an awkward movement for the operator and often results in additional mechanisms being needed in the transmission to obtain the more acceptable progressive shift schedule. However, with the cam, lever systems of the invention described herein by simply reversing the cam arrangement, a progressive shift schedule can be achieved regardless of the position of the gears or forks employed. For that matter, any pattern desired, such as might be required when using the same transmission in both left and right hand drive vehicles, can be obtained with minimum parts substitution.
An interlock mechanism is also provided in the present invention to prevent inadvertent selection of more than one gear ratio at a time. The interlock mechanism cooperates with the selector shaft to engage a selected cam upon movement of the selector shaft and thereby prevent actuation of non-selected levers during actuation of the selected lever. The mechanism is detented on the selector shaft to give the operator feel of the shift lever for each gear position. The mechanism is detented at the input in both the transverse and axial planes of the selector shaft to maximize "operator feel" at minimum expense, and avoid putting loads into the downstream components.
With the configuration of the invention described herein, the selector shaft can be brought out to either side of the transmission. Thus, left or right hand drive vehicles of either the conventional or COE configuration can readily be accommodated. Transverse placement of the shaft permits the use of actuating rods having minimum bends when utilized on the latter vehicles for remote control of shifting. Furthermore, placement of the selector shaft can be made with appropriate design modifications, in various desired planes and positions that may be deemed advantageous. For instance, the selector shaft could be placed in a vertical plane and the balance of the mechanism altered accordingly. Other advantages will become apparent from the detailed description of the preferred embodiment.